Mössbauer study of r.f. collapse in permalloy

The fast relaxation of hyperfine magnetic field forced by external r.f. field was studied for permalloy foils. The application of r.f. field caused a complete collapse of the six line Mössbauer spectrum to a single line. This effect was studied as a function of r.f. field intensity. Appearance of r.f. sidebands in collapse spectrum was observed.     

Mössbauer experiments in radio frequency magnetic fields: A method for investigations of nanostructured soft magnetic materials

We apply radio frequency (rf) effects, the sideband and the collapse effect, in the investigation of magnetic properties of nanostructured ferromagnetic alloys. We use the relative intensity of the sidebands in comparison to the central part of the spectrum to determine the relative samples' magnetostriction following successive preparation steps. Recent investigations of nanostructured soft ferromagnetic alloys in rf fields led to the discovery that the collapse in the Mössbauer spectra can become selective and partial. It means that the magnetisation reversal is not fast enough and varies differently in the various phases of the alloy. The application of rf magnetic fields then causes new kinds of rf forced relaxation‐type Mössbauer spectra. Experimental results and basic steps in the theoretical understanding are also presented.     

Frequency modulation of the 6.2 keV Mössbauer states of181Ta

Mössbauer sidebands up to the first order from a single parent line have been produced by subjecting a non magnetic W(¹⁸¹W) Mössbauer source to a strong oscillating magnetic field of up to 230 Oe amplitude and a frequency of about one megahertz. The sidebands positions and intensities agree very well with theory, which is based on a periodic time-dependent interaction of the magnetic field with the nuclear magnetic moments of ground and excited states, respectively. From the sideband intensity ag-factor ratio ofg _e /g _g =1.75(6) was derived.     

Mössbauer and its r.f. sideband effects in iron-rich glassy alloys

Room temperature Mössbauer and its r.f. induced sideband effects are studied in glassy Fe₄₀Ni₄₀P₁₄B₆ and Fe₈₀P₁₆C₃B₁ alloys in ribbon forms. The direction of the magnetic anisotropy is found to be in the plane of the ribbon for the first alloy while the latter has some out-of-plane anisotropy component. The 3d electronic configuration for iron in the two alloys seems to be about the same in light of the observed isomer shifts and the electron donor model. The r.f. induced sidebands in the spectra are attributed to the magnetostriction of the present alloys.     

Study on variation of magnetic anisotropy inside amorphous ribbons along thickness

Amorphous ribbons prepared by the melt spinning method exhibit some differences between the two surfaces and the bulk /1,2/. To determine the variation of the magnetic anisotropies, the Mössbauer spectra of the amorphous Fe48Ni31Si10B11 sample were recorded in an applied rf field. The surface of some samples were mechanical polished out. The collapse spectra of the amorphous samples showed some differences between the samples before and after polishing out the surface. It is in relation to the variation of magnetic anisotropy inside the ribbon along the thickness. A method to determine the spatial distribution of magnetic texture suggested by Xu et al/3/ was applied to this problem. The direction of the easy magnetization varies from one surface to the another surface.     

Investigation of the effect of annealing on the interface structure in Fe/Cr superlattices by conversion electron Mössbauer spectroscopy

The effect of vacuum annealing on the structure of the Fe/Cr superlattice, exhibiting giant magnetoresistance effect, has been investigated by Mössbauer spectroscopy. It is shown that annealing-induced activation of diffusion leads to a decrease in the thickness of “pure” Cr and Fe layers, redistribution of atoms in the interface regions, and a change in the giant magnetoresistance. The existence of the angular spatial dispersion of the magnetic anisotropy field at the interface has also been revealed.     

Mössbauer study of the separation of the rf sideband and collapse effects in invar

This report describes an investigation of the influence of coating of invar foil with various materials on the sideband and collapse effects caused by an applied r.f. magnetic field. It is shown that coating of invar foil with nonmetallic materials causes a decrease of the r.f. sideband effect without affecting the r.f. collapse effect. Coating with metallic materials reduces both r.f. effects because of screening due to the r.f. field. The separation of the r.f. sideband and collapse effects due to coating allows us to distinguish the different origins of these effects, and suggests that the r.f. sidebands are of magneto-acoustic origin, while the r.f. collapse is of purely magnetic origin.     

Mossbauer effect study of surface magnetization processes in iron

Magnetization processes at the surface of iron plates were investigated in a variable external field by means of Mössbauer emission and scattering spectroscopy of⁵⁷Fe. It is shown that the 90? walls of closure domains do not move until an externally applied field reaches a certain value. The “starting” values of the field determined from the Mössbauer measurements agree with those at which a sudden decrease in the growth rate of the bulk magnetization starts. Blocking of the surface domain wall movement occurs in a layer of at least 10 Μm thickness.     

Collapse in the Stoner-Wohlfarth noninteracting-particle model

A theory of Mössbauer spectra of noninteracting Stoner-Wohlfarth (SW) particles interacting with rf magnetic fields is developed. The theory makes it possible to calculate the absorption spectra for arbitrary frequency and amplitude of the rf field. The main features of the Stoner-Wohlfarth model are discussed. The Liouville superoperator formalism is used to generalize the results to the case of arbitrarily time-varying hyperfine fields at a nucleus. To understand the qualitative features of the collapse effect that are observed in the Mössbauer spectra of SW particles the particular case of a circularly polarized hyperfine field is studied, and an analytical expression is obtained describing the Mössbauer spectra for this case. An analysis is also made for weak rf magnetic fields and in this case the resonance behavior of the Mössbauer lines is traced as a function of the frequency of the rf field.     

Radio-frequency annealing effects in amorphous Fe40Ni40B20

The influence of radio frequency (rf) magnetic fields on the properties of amorphous Fe₄₀Ni₄₀B₂₀ was studied using Mössbauer spectroscopy. The measurements were performed with frequencies of 67 and 53 MHz and rf field intensity in the range of 1 to 12 Oe. The narrowing of the hyperfine spectra due to the rf field and the formation of rf sidebands were observed. The effect of instability and crystallization of the amorphous metal enhanced by the rf field at temperatures much lower than the crystallization temperature was observed.     

A Mössbauer study of spin-pinning in the oxide surface layer on fine iron particles

Fine α-Fe particles coated with an oxide surface layer were prepared by the evaporation technique. The spin pinning in the oxide surface layer were studied by means of the Mössbauer effect in an applied field of 6T during a thermal-cycle process. The anisotropy energy on the surface was estimated.     

The behaviour of superparamagnetic small particles in applied magnetic fields: A Mössbauer spectroscopic study of ferritin and haemosiderin

Small magnetically ordered particles display characteristic temperature dependent superparamagnetic behaviour in their Mössbauer spectra, as a result of the interplay between the thermal energy and the magnetic anisotropy energy of each particle. In the presence of an applied magnetic field there is an additional energy term arising from the net magnetic moment of the particles. The resulting Mössbauer spectra depend on the size of the anisotropy energy relative to the energy of the magnetic moment of the particles in the applied field. The present applied field Mössbauer measurements on ferritin and haemosiderin indicate that in these materials the anisotropy energy dominates, although the spectra can still be analysed to give information on the magnetic moment of the particles.     

Mössbauer study of FINEMET with different permeability

Stress field and magnetic field annealed FINEMET ribbons were investigated by ⁵⁷Fe Mössbauer spectroscopy, magnetic and XRD methods. The change in relative areas of the 2nd and 5th lines in the Mössbauer spectra indicated significant variation in magnetic anisotropy due to the different annealing. High velocity resolution Mössbauer spectroscopy was also used to control the model applied for the evaluation of Mössbauer spectra. A correlation was found between the permeability and the magnetic anisotropy of the annealed FINEMET samples. This can be applied to predict production parameters of FINEMET ribbons with more favorable soft magnetic properties for technological applications.     

High-frequency magnetic modulation of recoilless gamma radiation of57Fe

Radio-frequency (rf) magnetic modulation has been used to generate sidebands in⁵⁷Fe Mössbauer spectra of Fe_0.18Ni_0.82 Permalloy foils which have the smallest constant of magnetostriction among Fe?Ni alloys. Sidebands in Mössbauer spectra were observed at 30 MHz and 55 MHz. In addition to the generation of sidebands, the external rf magnetic field was found to alter the line positions of the original six line spectrum. An attempt was made to study acoustic vibrations in the foil by means of X-ray diffraction. The rf magnetic field caused changes in diffraction peak intensities and positions. It was found that X-ray diffraction can be used to study the amplitude of acoustic vibrations in Permalloy foils.     

Design of a conversion electron Mössbauer spectrometer based on an electron multiplier. Evaluation of the mean-escape-depth of the detected signals

A Conversion Electron Mössbauer Spectrometer to be used for the characterization of Fe-containing metal surfaces was designed and installed in an Ultra High Vacuum chamber. The design is based in the use of a Channeltron electron multiplier for the detection of electrons emerging from the sample after an incident γ-Ray is absorbed by resonant nuclear excitation. Using a Monte Carlo simulation for electron trajectories in solids the mean-escape-depth of the detected Mössbauer signal from a metallic iron sample was estimated to be 80 nm, assuming that the main signals being detected correspond to the Fe(M), Fe(L) and Fe(K) conversion electrons as well as the Fe(KLM), Fe(KLL) and Fe(LMM) Auger electrons. The sensitivity to the surface region was also estimated experimentally by acquiring Mössbauer spectra from a series of Fe films of different thickness deposited by magnetron sputtering on 304 stainless steel substrates.     

Ferrous ammonium sulphate hexahydrate Mössbauer revisited: a combined 57Fe Mössbauer and X-ray single crystal study

This paper investigates the electric-field gradient (efg) tensor for the Fe^2?+? ion in ferrous ammonium sulphate hexahydrate single crystals at 295 K from two different, but apparently equivalent, standpoints. Firstly, we calculate the expected angular dependence of the thin-crystal-limit reduced intensity ratios and total intensities of the quadrupole doublet utilizing earlier determined macroscopic intensity tensor data and X-ray-determined atomic-displacement parameters (ADPs) for the iron nucleus. The computer program used (MOSREF) utilizes matrix diagonalization in an arbitrary coordinate frame to calculate Mössbauer intensities for sites of any specified symmetry in crystals of any specified symmetry. Secondly we attempt to measure, directly by Mössbauer spectroscopy, the Lamb–Mössbauer factors that may, via the mean-squared-displacement tensor, be related to the Fe^2?+? ADPs. From the experimental angular dependence of the reduced intensities (areas), corrected to the thin crystal limit, and experimental Mössbauer-determined mean-squared displacements, we determine the microscopic electric-field-gradient tensors for the two symmetry-related sites in the unit cell. This is achieved using the refinement options of program MOSREF where reduced intensities and total intensities are refined simultaneously. Successful determination of microscopic tensors depends on anisotropy in the absorber recoilless fractions that are in turn related to anisotropic Lamb–Mössbauer factors.     

Mössbauer effect in small particles

Mössbauer spectroscopy is very sensitive to the special magnetic properties of ultrafine particles. Studies of particles in the superparamagnetic state allow determination of the particle size and the magnetic anisotropy energy constant A strong magnetic interaction between the particles may result in ordering of the magnetic moments of particles which would be superparamagnetic if they were non-interacting. This so-called superferromagnetic state can also be characterized by Mössbauer spectroscopy measurements. Furthermore, because a large fraction of the atoms in very small particles are in the surface layer, Mössbauer spectroscopy can be applied for studies of surface magnetism in small particles.     

A non-destructive analysis of the influence of sputter-etching on the magnetic surface properties of manganese zinc ferrite (100) surfaces using ellipsometry and the polar magneto-optical Kerr effect

Ellipsometry and the polar magneto-optical Kerr effect have been used in the photon energy range 0.65 eV?hv?5.4 eV to study sputter-etched manganese zinc ferrite (100) surfaces. With an annealed sample as a reference, a stratified media calculation indicates the presence of a paramagnetic surface layer of 8–40 nm thickness, depending on sputter parameters. Conversion electron Mössbauer spectroscopy and Rutherford backscattering spectroscopy fully confirmed these results.     

A study of the parameterisation of the uniaxial model of superparamagnetic relaxation

Mössbauer spectra of the iron storage protein ferritin taken over a range of temperatures exhibit the characteristic behaviour of superparamagnetic relaxation, and can be used to investigate a model for the superparamagnetic relaxation of a sample of small magnetic particles. A computer program is used to calculate a series of Mössbauer spectra based on the relaxation model, and these theoretical spectra are then fitted simultaneously to all the variable temperature experimental spectra, with the parameters of the uniaxial anisotropy model as the only variables.     

Magnetic enhancement in nano-sized Ni–Zn ferrite

Nanocrystalline Ni_0.35Zn_0.65Fe₂O₄ synthesized by mechanical alloying method and subsequent annealing at 300°C has been characterized by XRD, TEM and Mössbauer spectroscopic techniques. Mössbauer spectroscopic study divulges the enhancement of magnetic order, ordering temperature and magnetization in nano-crystalline sample compared to its bulk counterpart. This magnetic enhancement has mostly been prompted by cation redistribution in the nanosized sample. Zinc having strong A site affinity determines the nature and intensity of site exchange of cations, which has a strong influence in the genesis of enhancement/reduction in magnetic property of nano-crystalline Ni–Zn ferrite samples.